1. Field of the Invention
The present invention relates to a high-frequency power amplifier and, more particularly to improvement of a Doherty high-frequency power amplifier used with microwave-band and millimeterwave-band communications equipment for mobile communication, satellite communication or the like.
2. Description of the Related Art
In recent years, there has been an increasing demand for smaller, higher-output communications equipment used in a microwave band and a millimeter wave band. In addition, there has been also an increasing demand for higher quality of propagated signals. With this trend, needs for high-frequency power amplifiers with less distortion have been increasing.
Especially a microwave communications system using multi-carrier signals or recent modulated-wave signals based on the CDMA method or the like actuates an amplifier at an output level that is far lower than its maximum power rating in order to avoid influences of distortion caused by nonlinearity of the amplifier that amplifies signals.
Regardless of high frequencies, typical amplifiers usually have high input signal levels and provide higher efficiency toward maximum output levels of the amplifiers. If, however, an input signal level is sufficiently lower than a maximum output level, that is, if offset backoff (hereinafter referred to as “OBO”) is sufficiently large, then the efficiency is low accordingly. This has been making it difficult to achieve high efficiency.
The Doherty amplifier was first proposed by Doherty (“A New High Efficiency Power Amplifier For Modulated Waves”, Proceedings of the Institute of Radio Engineers, Vol. 24, No. 9, September, 1936).
The Doherty amplifier is intended to be used with an AM broadcasting transmitter for low to medium frequencies, and has a carrier amplifier and a peak amplifier that are connected by an impedance conversion line having an electrical length equivalent to a quarter wavelength of a signal frequency. This configuration permits dramatically improved efficiency at a low output level.
A report by Raab on theoretical values of efficiency obtained by the Doherty amplifier indicates that high efficiency is maintained at output levels from an output point, which is a quarter of a maximum output, to a maximum output point, and that the output level at which a highly efficient operation is performed can be lowered to a quarter or less of the maximum output by setting outputs of a peak amplifier greater than those of the carrier amplifier (“Efficiency of Doherty RF power-amplifier systems”, “IEEE Trans. Broadcast, vol. BC-33, pp. 77–83, September 1987).
There is a publicly known example wherein such Doherty amplifier is used in a microwave band. This has disclosed a Doherty amplifier equipped with a carrier amplifier that carries out higher harmonic load control and a peak amplifier that has a class B or class AB configuration as in the carrier amplifier and carries out higher harmonic load control (refer to, for example, paragraphs [0022] through [0024] and [0033] of Japanese Patent No. 2945833, and FIG. 4).
Another Doherty amplifier intended for achieving improved efficiency at a lower output level has been disclosed. This Doherty amplifier allows higher efficiency to be achieved from a low output level of one tenth or less of a maximum output, it can be achieved to triple an output of the carrier amplifier by making the size of a transistor used with its peak amplifier three times as large as the size of a transistor used with its carrier amplifier (refer to, for example, “An Extended Doherty Amplifier With High Efficiency Over a Wide Power Range” by M. Iwamoto et al., IEEE Trans. Microwave Theory Tech., Vol. 49, No. 12, pp. 2472–2479, December 2001).
Still another example of a Doherty amplifier intended for achieving improved efficiency at a lower output level has been disclosed. In this example, a plurality of peak amplifiers is used to provide an equivalent effect obtained by increasing the size of a transistor used with a peak amplifier (refer to, for example, “A Fully Matched N-Way Doherty Amplifier With Optimized Linearity” by Y. Yang et al., IEEE Trans. Microwave Theory Tech., Vol. 51, No. 3, pp. 986–993, March 2003).
A Doherty amplifier that adopts a parallel coupling configuration to improve linearity is disclosed in Published Japanese Translations of PCT International Publication for Patent Application No. H 10-513631.
Further, Japanese Patent Laid-open No. H 8-330873 has disclosed a configuration for linearly amplifying a noise-like RF signal having a multicarrier. The configuration includes a ¼ wavelength impedance transforming circuit that uses a load at an output end of a carrier amplifier as the normalized impedance of an optimal load impedance and a ½ wavelength phase shifter. In addition, an input end of a peak amplifier is disposed with a ¼ wavelength phase shifter, a ¼ wavelength impedance transforming circuit that uses a load at an output end of a peak amplifier as the normalized impedance of an optimal load impedance, and a ¼ wavelength phase shifter.
The Doherty amplifier disclosed in Japanese Patent No. 2945833 has a basic construction of a Doherty high-frequency power amplifier used with microwave-band or millimeterwave-band communications equipment. In response to a demand for an amplifier that restrains low distortion caused by an extended OBO and improves efficiency, the Doherty amplifier disclosed in M. Iwamoto et al. achieves higher efficiency by increasing the size of the transistor used with the peak amplifier. In this Doherty amplifier, however, a problem arises in that the carrier amplifier and the peak amplifier use transistors of significantly different sizes, so that its divider circuit and combiner circuit inevitably have complicated configurations.
Furthermore, a Doherty amplifier requires a Doherty network having an electrical length equivalent to a quarter wavelength of a signal frequency at an output end, and a phase compensating circuit at an input end, the phase compensating circuit having an electrical length equivalent to a quarter wavelength of a signal frequency for offsetting a phase difference between a carrier amplifier and a peak amplifier that occurs in the Doherty network. If operating frequencies are low, then these circuits inevitably become extremely large, resulting in an increased size of the whole amplifier. As a solution, therefore, the Doherty amplifier disclosed in Young et al. described above uses a plurality of phase compensating circuits and peak amplifiers to improve the efficiency at a lower output level. This, however, involves a complicated configuration. Furthermore, since a plurality of the phase compensating circuits and the peak amplifiers are provided, so that the phase compensating circuits take up even more area accordingly, making it difficult to accomplish a compact amplifier.